Helicase

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Structure of E. coli helicase RuvA (note that the heilcase core in RuvAB complex is RuvB and not RuvA and that RuvA alone do not show helicase activity) Helicase.png
Structure of E. coli helicase RuvA (note that the heilcase core in RuvAB complex is RuvB and not RuvA and that RuvA alone do not show helicase activity)

Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two hybridized nucleic acid strands (hence helic- + -ase ), using energy from ATP hydrolysis. There are many helicases, representing the great variety of processes in which strand separation must be catalyzed. Approximately 1% of eukaryotic genes code for helicases. [1]

Contents

The human genome codes for 95 non-redundant helicases: 64 RNA helicases and 31 DNA helicases. [2] Many cellular processes, such as DNA replication, transcription, translation, recombination, DNA repair, and ribosome biogenesis involve the separation of nucleic acid strands that necessitates the use of helicases. Some specialized helicases are also involved in sensing of viral nucleic acids during infection and fulfill an immunological function. A helicase is an enzyme that plays a crucial role in the DNA replication and repair processes. Its primary function is to unwind the double-stranded DNA molecule by breaking the hydrogen bonds between the complementary base pairs, allowing the DNA strands to separate. This creates a replication fork, which serves as a template for synthesizing new DNA strands. Helicase is an essential component of cellular mechanisms that ensures accurate DNA replication and maintenance of genetic information. DNA helicase catalyzes regression. RecG and the enzyme PriA work together to rewind duplex DNA, creating a Holliday junction. RecG releases bound proteins and the PriA helicase facilitates DNA reloading to resume DNA replication. RecG replaces the single-strand binding protein (SSB), which regulates the helicase-fork loading sites during fork regression. The SSB protein interacts with DNA helicases PriA and RecG to recover stalled DNA replication forks. These enzymes must bind to the SSB-helicase to be loaded onto stalled forks. Thermal sliding and DNA duplex binding are possibly supported by the wedge domain of RecG's association with the SSB linker. In a regression reaction facilitated by RecG and ATPHollidayjunctions are created for later processing.

Function

Helicases are often used to separate strands of a DNA double helix or a self-annealed RNA molecule using the energy from ATP hydrolysis, a process characterized by the breaking of hydrogen bonds between annealed nucleotide bases. They also function to remove nucleic acid-associated proteins and catalyze homologous DNA recombination. [3] Metabolic processes of RNA such as translation, transcription, ribosome biogenesis, RNA splicing, RNA transport, RNA editing, and RNA degradation are all facilitated by helicases. [3] Helicases move incrementally along one nucleic acid strand of the duplex with a directionality and processivity specific to each particular enzyme.

Helicases adopt different structures and oligomerization states. Whereas DnaB-like helicases unwind DNA as ring-shaped hexamers, other enzymes have been shown to be active as monomers or dimers. Studies have shown that helicases may act passively, waiting for uncatalyzed unwinding to take place and then translocating between displaced strands, [4] or can play an active role in catalyzing strand separation using the energy generated in ATP hydrolysis. [5] In the latter case, the helicase acts comparably to an active motor, unwinding and translocating along its substrate as a direct result of its ATPase activity. [6] Helicases may process much faster in vivo than in vitro due to the presence of accessory proteins that aid in the destabilization of the fork junction. [6]

DNA replication en.svg
Helicase (blue triangle) separates the intertwined DNA strands so that daughter strands can form.

Activation barrier in helicase activity

Enzymatic helicase action, such as unwinding nucleic acids is achieved through the lowering of the activation barrier () of each specific action. [7] [5] [8] [9] The activation barrier is a result of various factors, and can be defined by

where

  • = number of unwound base pairs (bps),
  • = free energy of base pair formation,
  • = reduction of free energy due to helicase, and
  • = reduction of free energy due to unzipping forces.

Factors that contribute to the height of the activation barrier include: specific nucleic acid sequence of the molecule involved, the number of base pairs involved, tension present on the replication fork, and destabilization forces. [7] [5] [8] [9]

Active and passive helicases

The size of the activation barrier to overcome by the helicase contributes to its classification as an active or passive helicase. In passive helicases, a significant activation barrier exists (defined as , where is the Boltzmann constant and is temperature of the system). Due to this significant activation barrier, its unwinding progression is affected largely by the sequence of nucleic acids within the molecule to unwind, and the presence of destabilization forces acting on the replication fork. [7] [5] [8] [9] Certain nucleic acid combinations will decrease unwinding rates (i.e. guanine and cytosine), while various destabilizing forces can increase the unwinding rate. [5] [8] [9] In passive systems, the rate of unwinding () is less than the rate of translocation () (translocation along the single-strand nucleic acid, ssNA), due to its reliance on the transient unraveling of the base pairs at the replication fork to determine its rate of unwinding. [7] [5] [8] [9]

In active helicases, , where the system lacks a significant barrier, as the helicase can destabilize the nucleic acids, unwinding the double-helix at a constant rate, regardless of the nucleic acid sequence. In active helicases, is closer to , due to the active helicase ability to directly destabilize the replication fork to promote unwinding. [7] [5] [8] [9]

Active helicases show similar behaviour when acting on both double-strand nucleic acids, dsNA, or ssNA, in regards to the rates of unwinding and rates of translocation, where in both systems and are approximately equal.

These two categories of helicases may also be modeled as mechanisms. In such models, the passive helicases are conceptualized as Brownian ratchets, driven by thermal fluctuations and subsequent anisotropic gradients across the DNA lattice. The active helicases, in contrast, are conceptualized as stepping motors – also known as powerstroke motors – utilizing either a conformational "inch worm" or a hand-over-hand "walking" mechanism to progress. [10] Depending upon the organism, such helix-traversing progress can occur at rotational speeds in the range of 5,000 [11] to 10,000 [12] R.P.M.

History of DNA helicases

DNA helicases were discovered in E. coli in 1976. This helicase was described as a "DNA unwinding enzyme" that is "found to denature DNA duplexes in an ATP-dependent reaction, without detectably degrading". [13] The first eukaryotic DNA helicase discovered was in 1978 in the lily plant. [14] Since then, DNA helicases were discovered and isolated in other bacteria, viruses, yeast, flies, and higher eukaryotes. [15] To date, at least 14 different helicases have been isolated from single celled organisms, 6 helicases from bacteriophages, 12 from viruses, 15 from yeast, 8 from plants, 11 from calf thymus, and approximately 25 helicases from human cells. [16] Below is a history of helicase discovery:

Structural features

The common function of helicases accounts for the fact that they display a certain degree of amino acid sequence homology; they all possess sequence motifs located in the interior of their primary structure, involved in ATP binding, ATP hydrolysis and translocation along the nucleic acid substrate. The variable portion of the amino acid sequence is related to the specific features of each helicase.

The presence of these helicase motifs allows putative helicase activity to be attributed to a given protein, but does not necessarily confirm it as an active helicase. Conserved motifs do, however, support an evolutionary homology among enzymes. Based on these helicase motifs, a number of helicase superfamilies have been distinguished.

Superfamilies

Helicases are classified in 6 groups (superfamilies) based on their shared sequence motifs. [26] Helicases not forming a ring structure are in superfamilies 1 and 2, and ring-forming helicases form part of superfamilies 3 to 6. [27] Helicases are also classified as α or β depending on if they work with single or double-strand DNA; α helicases work with single-strand DNA and β helicases work with double-strand DNA. They are also classified by translocation polarity. If translocation occurs 3’-5’ the helicase is type A; if translocation occurs 5’-3’ it is type B. [26]

All helicases are members of a P-loop, or Walker motif-containing family.

Helicase disorders and diseases

ATRX helicase mutations

The ATRX gene encodes the ATP-dependent helicase, ATRX (also known as XH2 and XNP) of the SNF2 subgroup family, that is thought to be responsible for functions such as chromatin remodeling, gene regulation, and DNA methylation. [31] [32] [33] [34] These functions assist in prevention of apoptosis, resulting in cortical size regulation, as well as a contribution to the survival of hippocampal and cortical structures, affecting memory and learning. [31] This helicase is located on the X chromosome (Xq13.1-q21.1), in the pericentromeric heterochromatin and binds to heterochromatin protein 1. [31] [33] Studies have shown that ATRX plays a role in rDNA methylation and is essential for embryonic development. [35] Mutations have been found throughout the ATRX protein, with over 90% of them being located in the zinc finger and helicase domains. [36] Mutations of ATRX can result in X-linked-alpha-thalassaemia-mental retardation (ATR-X syndrome). [31]

Various types of mutations found in ATRX have been found to be associated with ATR-X, including most commonly single-base missense mutations, as well as nonsense, frameshift, and deletion mutations. [34] Characteristics of ATR-X include: microcephaly, skeletal and facial abnormalities, mental retardation, genital abnormalities, seizures, limited language use and ability, and alpha-thalassemia. [31] [35] [32] The phenotype seen in ATR-X suggests that the mutation of ATRX gene causes the downregulation of gene expression, such as the alpha-globin genes. [32] It is still unknown what causes the expression of the various characteristics of ATR-X in different patients. [35]

XPD helicase point mutations

XPD (Xeroderma pigmentosum factor D, also known as protein ERCC2) is a 5'-3', Superfamily II, ATP-dependent helicase containing iron-sulphur cluster domains. [26] [37] Inherited point mutations in XPD helicase have been shown to be associated with accelerated aging disorders such as Cockayne syndrome (CS) and trichothiodystrophy (TTD). [38] Cockayne syndrome and trichothiodystrophy are both developmental disorders involving sensitivity to UV light and premature aging, and Cockayne syndrome exhibits severe mental retardation from the time of birth. [38] The XPD helicase mutation has also been implicated in xeroderma pigmentosum (XP), a disorder characterized by sensitivity to UV light and resulting in a several 1000-fold increase in the development of skin cancer. [38]

XPD is an essential component of the TFIIH complex, a transcription and repair factor in the cell. [38] [39] [40] [41] [42] As part of this complex, it facilitates nucleotide excision repair by unwinding DNA. [38] TFIIH assists in repairing damaged DNA such as sun damage. [38] [39] [40] [41] [42] A mutation in the XPD helicase that helps form this complex and contributes to its function causes the sensitivity to sunlight seen in all three diseases, as well as the increased risk of cancer seen in XP and premature aging seen in trichothiodystrophy and Cockayne syndrome. [38]

XPD helicase mutations leading to trichothiodystrophy are found throughout the protein in various locations involved in protein-protein interactions. [38] This mutation results in an unstable protein due to its inability to form stabilizing interactions with other proteins at the points of mutations. [38] This, in turn, destabilizes the entire TFIIH complex, which leads to defects with transcription and repair mechanisms of the cell. [38]

It has been suggested that XPD helicase mutations leading to Cockayne syndrome could be the result of mutations within XPD, causing rigidity of the protein and subsequent inability to switch from repair functions to transcription functions due to a "locking" in repair mode. [38] This could cause the helicase to cut DNA segments meant for transcription. [38] Although current evidence points to a defect in the XPD helicase resulting in a loss of flexibility in the protein in cases of Cockayne syndrome, it is still unclear how this protein structure leads to the symptoms described in Cockayne syndrome. [38]

In xeroderma pigmentosa, the XPD helicase mutation exists at the site of ATP or DNA binding. [38] This results in a structurally functional helicase able to facilitate transcription, however it inhibits its function in unwinding DNA and DNA repair. [38] The lack of a cell's ability to repair mutations, such as those caused by sun damage, is the cause of the high cancer rate in xeroderma pigmentosa patients.

RecQ family mutations

RecQ helicase RecQ helicase.png
RecQ helicase

RecQ helicases (3'-5') belong to the Superfamily II group of helicases, which help to maintain stability of the genome and suppress inappropriate recombination. [43] [44] Deficiencies and/or mutations in RecQ family helicases display aberrant genetic recombination and/or DNA replication, which leads to chromosomal instability and an overall decreased ability to proliferate. [43] Mutations in RecQ family helicases BLM, RECQL4, and WRN, which play a role in regulating homologous recombination, have been shown to result in the autosomal recessive diseases Bloom syndrome (BS), Rothmund–Thomson syndrome (RTS), and Werner syndrome (WS), respectively. [44] [45]

Bloom syndrome is characterized by a predisposition to cancer with early onset, with a mean age-of-onset of 24 years. [44] [46] Cells of Bloom syndrome patients show a high frequency of reciprocal exchange between sister chromatids (SCEs) and excessive chromosomal damage. [47] There is evidence to suggest that BLM plays a role in rescuing disrupted DNA replication at replication forks. [47]

Werner syndrome is a disorder of premature aging, with symptoms including early onset of atherosclerosis and osteoporosis and other age related diseases, a high occurrence of sarcoma, and death often occurring from myocardial infarction or cancer in the 4th to 6th decade of life. [44] [48] Cells of Werner syndrome patients exhibit a reduced reproductive lifespan with chromosomal breaks and translocations, as well as large deletions of chromosomal components, causing genomic instability. [48]

Rothmund-Thomson syndrome, also known as poikiloderma congenitale, is characterized by premature aging, skin and skeletal abnormalities, rash, poikiloderma, juvenile cataracts, and a predisposition to cancers such as osteosarcomas. [44] [49] Chromosomal rearrangements causing genomic instability are found in the cells of Rothmund-Thomson syndrome patients. RecQ is a family of DNA helicase enzymes that are found in various organisms including bacteria, archaea, and eukaryotes (like humans). These enzymes play important roles in DNA metabolism during DNA replication, recombination, and repair. There are five known RecQ helicase proteins in humans: RecQ1, BLM, WRN, RecQ4, and RecQ5. Mutations in some of these genes are associated with genetic disorders. For instance, mutations in the BLM gene cause Bloom syndrome, which is characterized by increased cancer risk and other health issues. [50] Mutations in the WRN gene lead to Werner syndrome, a condition characterized by premature aging and an increased risk of age-related diseases. RecQ helicases are crucial for maintaining genomic stability and integrity. They help prevent the accumulation of genetic abnormalities that can lead to diseases like cancer. Genome integrity depends on the RecQ DNA helicase family, which includes DNA repair, recombination, replication, and transcription processes. Genome instability and early aging are conditions that arise from mutations in human RecQ helicases. [51] RecQ helicase Sgs1 is missing in yeast cells, making them useful models for comprehending human cell abnormalities and the RecQ helicase function. [52] The RecQ helicase family member, RECQ1, is connected to a small number of uncommon genetic cancer disorders in individuals. It participates in transcription, the cell cycle, and DNA repair. According to recent research, missense mutations in the RECQ1 gene may play a role in the development of familial breast cancer. DNA helicases are frequently attracted to regions of DNA damage and are essential for cellular DNA replication, recombination, repair, and transcription. Chemical manipulation of their molecular processes can change the rate at which cancer cells divide, as well as, the efficiency of transactions and cellular homeostasis. Small-molecule-induced entrapment of DNA helicases, a type of DNA metabolic protein, may have deleterious consequences on rapidly proliferating cancer cells, which could be effective in cancer treatment.

During meiosis DNA double-strand breaks and other DNA damages in a chromatid are repaired by homologous recombination using either the sister chromatid or a homologous non-sister chromatid as template. This repair can result in a crossover (CO) or, more frequently, a non-crossover (NCO) recombinant. In the yeast Schizosaccharomyces pombe the FANCM-family DNA helicase FmI1 directs NCO recombination formation during meiosis. [53] The RecQ-type helicase Rqh1 also directs NCO meiotic recombination. [54] These helicases, through their ability to unwind D-loop intermediates, promote NCO recombination by the process of synthesis-dependent strand annealing.

In the plant Arabidopsis thaliana , FANCM helicase promotes NCO and antagonizes the formation of CO recombinants. [55] Another helicase, RECQ4A/B, also independently reduces COs. It was suggested that COs are restricted because of the long term costs of CO recombination, that is, the breaking up of favourable genetic combinations of alleles built up by past natural selection. [55]

RNA helicases

Human DEAD-box RNA helicase Human DEAD-box RNA helicase.jpg
Human DEAD-box RNA helicase
This image represents the different promoter sequences and accessory domains that aid in RNA unwinding (local strand separation). The regions in red are ATP binding domains and the regions in yellow are RNA interaction domains. Specific sequences termed DEAD box proteins are also present that help catalyze reactions in which ATP does not need to be directly hydrolyzed, as long as it binds to the domains on the strand. DEAD box Accessory Domains.png
This image represents the different promoter sequences and accessory domains that aid in RNA unwinding (local strand separation). The regions in red are ATP binding domains and the regions in yellow are RNA interaction domains. Specific sequences termed DEAD box proteins are also present that help catalyze reactions in which ATP does not need to be directly hydrolyzed, as long as it binds to the domains on the strand.

RNA helicases are essential for most processes of RNA metabolism such as ribosome biogenesis, pre-mRNA splicing, and translation initiation. They also play an important role in sensing viral RNAs. [56] RNA helicases are involved in the mediation of antiviral immune response because they can identify foreign RNAs in vertebrates. About 80% of all viruses are RNA viruses and they contain their own RNA helicases. [57] Defective RNA helicases have been linked to cancers, infectious diseases and neuro-degenerative disorders. [56] Some neurological disorders associated with defective RNA helicases are: amyotrophic lateral sclerosis, spinal muscular atrophy, spinocerebellar ataxia type-2, Alzheimer disease, and lethal congenital contracture syndrome. [57]

RNA helicases and DNA helicases can be found together in all the helicase superfamilies except for SF6. [58] [59] All the eukaryotic RNA helicases that have been identified up to date are non-ring forming and are part of SF1 and SF2. On the other hand, ring-forming RNA helicases have been found in bacteria and viruses. [56] However, not all RNA helicases exhibit helicase activity as defined by enzymatic function, i.e., proteins of the Swi/Snf family. Although these proteins carry the typical helicase motifs, hydrolize ATP in a nucleic acid-dependent manner, and are built around a helicase core, in general, no unwinding activity is observed. [60]

RNA helicases that do exhibit unwinding activity have been characterized by at least two different mechanisms: canonical duplex unwinding and local strand separation. Canonical duplex unwinding is the stepwise directional separation of a duplex strand, as described above, for DNA unwinding. However, local strand separation occurs by a process wherein the helicase enzyme is loaded at any place along the duplex. This is usually aided by a single-strand region of the RNA, and the loading of the enzyme is accompanied with ATP binding. [61] Once the helicase and ATP are bound, local strand separation occurs, which requires binding of ATP but not the actual process of ATP hydrolysis. [62] Presented with fewer base pairs the duplex then dissociates without further assistance from the enzyme. This mode of unwinding is used by the DEAD/DEAH box helicases. [63]

An RNA helicase database [64] is currently available online that contains a comprehensive list of RNA helicases with information such as sequence, structure, and biochemical and cellular functions. [56]

Diagnostic tools for helicase measurement

Measuring and monitoring helicase activity

Various methods are used to measure helicase activity in vitro. These methods range from assays that are qualitative (assays that usually entail results that do not involve values or measurements) to quantitative (assays with numerical results that can be utilized in statistical and numerical analysis). In 1982–1983, the first direct biochemical assay was developed for measuring helicase activity. [15] [65] This method was called a "strand displacement assay".

Other methods were later developed that incorporated some, if not all of the following: high-throughput mechanics, the use of non-radioactive nucleotide labeling, faster reaction time/less time consumption, real-time monitoring of helicase activity (using kinetic measurement instead of endpoint/single point analysis). These methodologies include: "a rapid quench flow method, fluorescence-based assays, filtration assays, a scintillation proximity assay, a time resolved fluorescence resonance energy transfer assay, an assay based on flashplate technology, homogenous time-resolved fluorescence quenching assays, and electrochemiluminescence-based helicase assays". [16] With the use of specialized mathematical equations, some of these assays can be utilized to determine how many base paired nucleotides a helicase can break per hydrolysis of 1 ATP molecule. [66]

Commercially available diagnostic kits are also available. One such kit is the "Trupoint" diagnostic assay from PerkinElmer, Inc. This assay is a time-resolved fluorescence quenching assay that utilizes the PerkinElmer "SignalClimb" technology that is based on two labels that bind in close proximity to one another but on opposite DNA strands. One label is a fluorescent lanthanide chelate, which serves as the label that is monitored through an adequate 96/384 well plate reader. The other label is an organic quencher molecule. The basis of this assay is the "quenching" or repressing of the lanthanide chelate signal by the organic quencher molecule when the two are in close proximity – as they would be when the DNA duplex is in its native state. Upon helicase activity on the duplex, the quencher and lanthanide labels get separated as the DNA is unwound. This loss in proximity negates the quenchers ability to repress the lanthanide signal, causing a detectable increase in fluorescence that is representative of the amount of unwound DNA and can be used as a quantifiable measurement of helicase activity. The execution and use of single-molecule fluorescence imaging techniques, focusing on methods that include optical trapping in conjunction with epifluorescent imaging, and also surface immobilization in conjunction with total internal reflection fluorescence visualization. Combined with microchannel flow cells and microfluidic control, allow individual fluorescently labeled protein and DNA molecules to be imaged and tracked, affording measurement of DNA unwinding and translocation at single-molecule resolution. [67]

Determining helicase polarity

Helicase polarity, which is also deemed "directionality", is defined as the direction (characterized as 5'→3' or 3'→5') of helicase movement on the DNA/RNA single-strand along which it is moving. This determination of polarity is vital in f.ex. determining whether the tested helicase attaches to the DNA leading strand, or the DNA lagging strand. To characterize this helicase feature, a partially duplex DNA is used as the substrate that has a central single-strand DNA region with different lengths of duplex regions of DNA (one short region that runs 5'→3' and one longer region that runs 3'→5') on both sides of this region. [68] Once the helicase is added to that central single-strand region, the polarity is determined by characterization on the newly formed single-strand DNA.

See also

Related Research Articles

<span class="mw-page-title-main">DNA replication</span> Biological process

In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all living organisms acting as the most essential part of biological inheritance. This is essential for cell division during growth and repair of damaged tissues, while it also ensures that each of the new cells receives its own copy of the DNA. The cell possesses the distinctive property of division, which makes replication of DNA essential.

<span class="mw-page-title-main">Polymerase</span> Class of enzymes which synthesize nucleic acid chains or polymers

In biochemistry, a polymerase is an enzyme that synthesizes long chains of polymers or nucleic acids. DNA polymerase and RNA polymerase are used to assemble DNA and RNA molecules, respectively, by copying a DNA template strand using base-pairing interactions or RNA by half ladder replication.

<span class="mw-page-title-main">RecBCD</span> Family of protein complexes in bacteria

Exodeoxyribonuclease V is an enzyme of E. coli that initiates recombinational repair from potentially lethal double strand breaks in DNA which may result from ionizing radiation, replication errors, endonucleases, oxidative damage, and a host of other factors. The RecBCD enzyme is both a helicase that unwinds, or separates the strands of DNA, and a nuclease that makes single-stranded nicks in DNA. It catalyses exonucleolytic cleavage in either 5′- to 3′- or 3′- to 5′-direction to yield 5′-phosphooligonucleotides.

RecQ helicase is a family of helicase enzymes initially found in Escherichia coli that has been shown to be important in genome maintenance. They function through catalyzing the reaction ATP + H2O → ADP + P and thus driving the unwinding of paired DNA and translocating in the 3' to 5' direction. These enzymes can also drive the reaction NTP + H2O → NDP + P to drive the unwinding of either DNA or RNA.

<span class="mw-page-title-main">Nucleotide excision repair</span> DNA repair mechanism

Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals, radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucleotide excision repair (NER), base excision repair (BER), and DNA mismatch repair (MMR). While the BER pathway can recognize specific non-bulky lesions in DNA, it can correct only damaged bases that are removed by specific glycosylases. Similarly, the MMR pathway only targets mismatched Watson-Crick base pairs.

<span class="mw-page-title-main">Homologous recombination</span> Genetic recombination between identical or highly similar strands of genetic material

Homologous recombination is a type of genetic recombination in which genetic information is exchanged between two similar or identical molecules of double-stranded or single-stranded nucleic acids.

<span class="mw-page-title-main">Werner syndrome helicase</span> Enzyme found in humans

Werner syndrome ATP-dependent helicase, also known as DNA helicase, RecQ-like type 3, is an enzyme that in humans is encoded by the WRN gene. WRN is a member of the RecQ Helicase family. Helicase enzymes generally unwind and separate double-stranded DNA. These activities are necessary before DNA can be copied in preparation for cell division. Helicase enzymes are also critical for making a blueprint of a gene for protein production, a process called transcription. Further evidence suggests that Werner protein plays a critical role in repairing DNA. Overall, this protein helps maintain the structure and integrity of a person's DNA.

<span class="mw-page-title-main">Replisome</span> Molecular complex

The replisome is a complex molecular machine that carries out replication of DNA. The replisome first unwinds double stranded DNA into two single strands. For each of the resulting single strands, a new complementary sequence of DNA is synthesized. The total result is formation of two new double stranded DNA sequences that are exact copies of the original double stranded DNA sequence.

<span class="mw-page-title-main">Holliday junction</span> Branched nucleic acid structure

A Holliday junction is a branched nucleic acid structure that contains four double-stranded arms joined. These arms may adopt one of several conformations depending on buffer salt concentrations and the sequence of nucleobases closest to the junction. The structure is named after Robin Holliday, the molecular biologist who proposed its existence in 1964.

<span class="mw-page-title-main">ERCC2</span> Mammalian protein found in humans

TFIIH subunit XPD is a protein that in humans is encoded by the ERCC2 gene. It is a component of the general transcription and DNA repair factor IIH (TFIIH) core complex involved in transcription-coupled nucleotide excision repair.

Transcription factor II H (TFIIH) is an important protein complex, having roles in transcription of various protein-coding genes and DNA nucleotide excision repair (NER) pathways. TFIIH first came to light in 1989 when general transcription factor-δ or basic transcription factor 2 was characterized as an indispensable transcription factor in vitro. This factor was also isolated from yeast and finally named TFIIH in 1992.

<span class="mw-page-title-main">Bloom syndrome protein</span> Mammalian protein found in humans

Bloom syndrome protein is a protein that in humans is encoded by the BLM gene and is not expressed in Bloom syndrome.

PcrA, standing for plasmid copy reduced is a helicase that was originally discovered in a screen for chromosomally encoded genes that are affected in plasmid rolling circle replication in the Gram-positive pathogen Staphylococcus aureus.

<span class="mw-page-title-main">RECQL</span> Protein-coding gene in the species Homo sapiens

ATP-dependent DNA helicase Q1 is an enzyme that in humans is encoded by the RECQL gene.

Genome instability refers to a high frequency of mutations within the genome of a cellular lineage. These mutations can include changes in nucleic acid sequences, chromosomal rearrangements or aneuploidy. Genome instability does occur in bacteria. In multicellular organisms genome instability is central to carcinogenesis, and in humans it is also a factor in some neurodegenerative diseases such as amyotrophic lateral sclerosis or the neuromuscular disease myotonic dystrophy.

<span class="mw-page-title-main">Synthesis-dependent strand annealing</span>

Synthesis-dependent strand annealing (SDSA) is a major mechanism of homology-directed repair of DNA double-strand breaks (DSBs). Although many of the features of SDSA were first suggested in 1976, the double-Holliday junction model proposed in 1983 was favored by many researchers. In 1994, studies of double-strand gap repair in Drosophila were found to be incompatible with the double-Holliday junction model, leading researchers to propose a model they called synthesis-dependent strand annealing. Subsequent studies of meiotic recombination in S. cerevisiae found that non-crossover products appear earlier than double-Holliday junctions or crossover products, challenging the previous notion that both crossover and non-crossover products are produced by double-Holliday junctions and leading the authors to propose that non-crossover products are generated through SDSA.

Progeroid syndromes (PS) are a group of rare genetic disorders that mimic physiological aging, making affected individuals appear to be older than they are. The term progeroid syndrome does not necessarily imply progeria, which is a specific type of progeroid syndrome.

<span class="mw-page-title-main">Protein ZGRF1</span> Protein-coding gene in the species Homo sapiens

Protein ZGRF1 is a protein encoded in the human by the ZGRF1 gene also known as C4orf21, that has a weight of 236.6 kDa. The ZGRF1 gene product localizes to the cell nucleus and promotes DNA repair by stimulating homologous recombination. This gene shows relatively low expression in most human tissues, with increased expression in situations of chemical dependence. ZGRF1 is orthologous to nearly all eukaryotes. Functional domains of this protein link it to a series of helicases, most notably the AAA_12 and AAA_11 domains.

<span class="mw-page-title-main">DNA base flipping</span> Biochemical process

DNA base flipping, or nucleotide flipping, is a mechanism in which a single nucleotide base, or nucleobase, is rotated outside the nucleic acid double helix. This occurs when a nucleic acid-processing enzyme needs access to the base to perform work on it, such as its excision for replacement with another base during DNA repair. It was first observed in 1994 using X-ray crystallography in a methyltransferase enzyme catalyzing methylation of a cytosine base in DNA. Since then, it has been shown to be used by different enzymes in many biological processes such as DNA methylation, various DNA repair mechanisms, and DNA replication. It can also occur in RNA double helices or in the DNA:RNA intermediates formed during RNA transcription.

<span class="mw-page-title-main">Helicase, POLQ-like</span> Protein-coding gene in the species Homo sapiens

Helicase, POLQ-like, also known as Helicase Q (HELQ), HEL308 and Holliday junction migration protein, encoded by the gene HELQ1, is a DNA helicase found in humans, archea and many other organisms.

References

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